Chlorinated dibenzo-p-dioxins (CDDs) and dibenzofurans (CDFs) are found worldwide as environmental pollutants. They have no known industrial use but are produced during the synthesis and breakdown of other chlorinated organic chemicals. They have been involved in several incidents of human poisoning. Brominated dioxins and furans are also suspected of posing an occupational and environmental hazard. Brominated napthalenes, a related group of compounds, were involved in an incident of human and animal poisoning in the past decade. The degree of halogenation correlates with the toxicity of these compounds, with those congeners possessing less than four halogens being relatively non- toxic. Halogenation in the four lateral positions is required for toxicity which then tends to decrease as the number of halogens increases. Metabolism of these chemicals is a detoxification process. Persistence can result in increasing body burdens upon repeated low dose exposures. While oral absorption is quite good (>70%), dermal absorption tends to be limited. However, as doses are decreased towards the levels which exist in environmental situations, as much as 40-50% of the applied dose may be absorbed through the skin. After a single application, absorption continued for several days at a slow rate. 2,3,7,8-TetraCDF (TCDF), which is metabolized relatively rapidly, was better absorbed than the related compounds, 2,3,4,7,8-pentaCDF, 1,2,3,7,8-pentaCDF, or 2,3,7,8-tetraCDD (TCDD). The disposition of 1,2,3,7,-pentaCDF was examined in male rats. This isomer was distributed throughout the body (liver and adipose were the major tissue depots), and eliminated in the feces with a half-life of 2 days. Metabolism occurred prior to biliary excretion. These results are similar to those obtained with TCDF but are in contrast to those from studies with 2,3,4,7,8-pentaCDF and TCDD which are both poorly metabolized, and therefore persistent.